1. Field of the Invention
The present invention relates to the field of phase-locked loops (PLLs), and more particularly to a resampling filter in an analog PLL. The novel filter is especially suitable for use in integrated circuit designs.
2. Description of Related Art
A filter in an analog loop for a PLL must satisfy a number of requirements. It should provide a limited impedance into which the charge pump pumps its charge. This impedance should be stable, and preferably be at a constant voltage. This makes the charge pump repeatable, which is of importance for the matching in the signal shaping. It should retime an irregular (or semi-regular) signal from the charge pump into an equidistant signal. The irregularity is created by the mistiming of the reference signal. Thus, the signal out of the charge pump is not only variable in effective magnitude, but also in phase. The timing problem creates phase modulated spectra on the charge pump output. The related Bessel functions carry the signal down to near DC. Retiming is effective in repressing this again. The filter must provide a normal low pass filter type of first order or even higher. For stability the higher orders may become active relatively late. The impedance of the VCO that is controlled by the filter should be considered infinite. The filter must be such that the loop is unconditionally stable. The rest of the loop will have one dominant pole (VCO). The filter must be low noise. It should also not introduce more errors than resolved in the rest of the system.
FIG. 1 shows a conventional traditional PLL filter with a resistor and a capacitor. Although this is generally thought of a normal low pass filter, it is in fact really a pole and a zero in series with each other.
The control on the current source is in fact the charge up/down enable signal. Thus the current out of the current source is in fact the actual charge current. FIG. 2 shows the voltage output of such a filter.
At a certain moment the charge pump starts pumping. At that moment the voltage jumps; the current will lead to a voltage across the resistor. Once the current stops this jump is made back again. The capacitor will meanwhile start charging, which is depicted in the slanting part of the line, giving a different voltage between the start and the end. In effect, the capacitor provides an integrating part in the filter, and the resistor a proportional part.
The integrated part is relatively straightforward. However, the resistor contribution is quite complex. The resistor does not affect the output of the filter once the charge pump has completed it pumping action. Thus, the effect of the resistor must come from the ‘jump part’ only. It is this jump voltage that makes the difference between a stable and an unstable loop. In a PLL, the contribution of the resistor after the VCO will in fact be the integral (VCO) of the area which comes from the resistor. It must be the area; both time and magnitude matter. The time defines the period over which the VCO will run extra fast (or slow). The resistor defines the actual voltage, together with the charge current. Expressed in system terms, the resistor size will make the system less or more stable, the time domain carries the linear nature of the error signal. It is however undesirable to employ resistors in integrated circuits.